In recent years, there has been increasing demand for optical transmission systems that are capable of 40 Gigabit/second (Gbps) or 100 Gbps. In order to achieve such optical transmission systems, there have been attempts to adopt various types of modulation that offer superior efficiency in utilizing the frequency or superior Signal-to-Noise Ratio (SNR) tolerance. In particular, multi-level modulation capable of transmitting multiple-bit information in one symbol time is receiving attention. For example, a technique in which multi-level phase modulation and polarization multiplexing are combined, a technique for performing multi-level quadrature amplitude modulation with the combination of phase and amplitude, or the like are being actively studied.
In optical transmission systems in which the above-described multi-level modulation is adopted, there are some cases in which a signal decoded at the receiving end is received in a state different from that of the original signal modulated at the transmitting end depending on the operating environment of the system, such as the state of transmission lines. For this reason, it is known that the transmission data may not be received correctly due to the occurrence of the logic inversion and a swapping of bits in one symbol time.
As a related technique, a method for detecting and compensating for the logic inversion and swapping of bits has been proposed. According to this method, in an optical transmission system with multi-level modulation using polarization multiplexing, a detecting bit with a specific pattern which is set according to the number of bits to be transmitted in one symbol time is given to a transmission signal, and an optical signal which is modulated according to the transmission signal is transmitted from an optical transmitter to a transmission line. The optical receiver detects and compensates for the logic inversion or bit swap (the swapping of bits) of the received data by using the detecting bit included in a received signal. (For example, see Japanese Laid-open Patent Publication No. 2009-89194)
As another related technique for processing a transmission signal and a reception signal, a method for rearranging the data stored in a specific frame into a plurality of logical lanes and performing signal processing on each of the logical lanes is known. In this method, for example, when the data of a plurality of frames is divided into logical lanes, the data is rearranged while rotating the logical lanes. (For example, see U.S. Pat. No. 7,362,779)
In the above-mentioned compensation method in the related art, a pattern of detecting bits is configured so as not to be dependent upon a logic inversion and a bit swap, and by detecting a pattern which is not dependent upon a logic inversion at the receiving end, it becomes possible to detect the occurrence of a bit swap. Moreover, by detecting a pattern that is not dependent upon a bit swap at the receiving end, it also becomes possible to detect the occurrence of a logic inversion. However, such processing of a detecting bit at the receiving end, i.e., the process of detecting a pattern which is not dependent upon one of a logic inversion or a bit swap in order to detect the occurrence of the other one of a logic inversion or a bit swap, may cause a logic inversion and a bit swap on the detecting bit itself, and thus the processing time tends to be longer. The above-mentioned signal processing with frame rearrangement helps to reduce the processing time, but does not necessarily prevent the logic inversion and bit swap of the detecting bit itself from occurring. For this reason, the conventional compensation technique has a limitation in detecting and compensating for the logic inversion and bit swap occurring to the reception data at an early stage.
The above-mentioned compensation system in the related art is intended for an optical transmission system with multi-level modulation using polarization multiplexing, but the logic inversion and swapping of bits caused to the reception data depending on the operating environment of the system such as the state of transmission lines may also occur in cases where the optical signal of the multi-level modulation not using polarization multiplexing is transmitted and received. Irrespective of whether or not the polarization multiplexing is used, it is a common problem with optical transmission systems with multi-level modulation that the logic inversion and the bit swap need to be detected at an early stage and compensated for.
In order to deal with the above-discussed problem, the Applicant has disclosed in Japanese Patent Application No. 2009-146056 (hereinafter, this may be simply referred to as “the invention of the prior application”), that the data stored in a specified frame is rearranged into a plurality of logical lanes such that the speed of the signal processing will increase, and that the lane ID, which is used to detect what logical lane out of the logical lanes the start of the data string of a frame is arranged in after the data string is rearranged, is assigned to a non-scrambled area within an overhead part of the frame to transmit the optical signal of the multi-level modulation. According to the invention of the prior application, it is possible to detect an inversion of bits and a swapping of lanes which occur to the reception data depending on the operating environment of the system at an early state by using the lane ID, and it is also possible to compensate for the detected inversion of bits and the swapping of lanes.
However, the invention of the prior application still has a problem in which it is difficult to precisely detect the lane ID at the receiving end when a bit error can occur at a relatively high probability, for example when an optical signal is transmitted at an even higher speed.
The problem of the invention of the prior application will be described in detail. In general optical transmission systems to which the invention of the prior application may be applied, a transmitter unit modulates light in accordance with a data signal to which a FEC (Forward Error Correction) code is added, and the optical signal is transmitted to a receiver unit via a transmission line. The receiver unit receives an optical signal through the transmission line, and performs an error correction by using the FEC code contained in the decoded reception data. The error correction is performed after the data strings are rearranged and the frame is regenerated according to a result of the detection of the lanes of the reception data. For this reason, a desired level of detection accuracy is required for the detection of the lane ID, which is contained in the reception data according to the invention of the prior application, before the error correction is performed by using the FEC code, i.e., on the condition that the bit error rate (BER) is relatively high. However, a sequence number (or continuous values) corresponding to the frame numbers are assigned to the lane IDs according to the invention of the prior application. Thus, when a bit error occurs in the data representing the sequence number, the arrangement of the logical lane is erroneously detected.